In U.S. Pat. No. 4,539,169 there is described an extrusion apparatus for forming a co-extrusion from extruded strips. In such arrangement there is provided a plurality of individual extrusion devices, each terminating in an extrusion head and extruding a profile strip onto a single common rotating roller. The upper surface of the nozzle outlet of each extrusion head is in the form of an extrusion bar which has a shape which corresponds with that of the upper portion of the tread strip to be produced. The lower surface of the nozzle outlet is formed by the rotating roller.
The rotation of the roller promotes the removal of each of the component profile strips from which the tread strip is produced, from the extrusion head, whereby the extrusion pressure in the head can be reduced. There are considerable advantages to be gained if the extrusion head can be operated at lower extrusion pressures. For example, the means for sealing the heads can be simplified. More especially, in conventional arrangement, high extrusion pressures of up to 200 bars exist in the extrusion heads and, accordingly, there is a risk of undesirably introducing shearing energy into the rubber mixtures, which are somewhat temperature-sensitive. This risk is substantially reduced in the arrangement described in U.S. Patent Specification No. 4,539,169. Furthermore, the roller exerts an additional calibrating effect upon the tread strip portion, whereby the accuracy of the finished tire profile is increased.
However, such a single roller system has disadvantages, particularly if a tread strip, formed from a plurality of rubber mixtures having substantially different properties, such as viscosities, from one another, is to be produced.
Since the lower surface of the nozzle outlet is formed by the rotating roller, this means that there is a small, but significant spacing between the nozzle roller. Accordingly, the extruded profile must always be extruded onto the roller in a width wider than is actually needed. This is because, otherwise, the full thickness in the edge regions of the relatively thin profile strip would not be extruded. The excess width is removed by an edge trimming operation, which is known in technical circles as "flashing" in consequence, the edges of the profile strips need subsequently to be trimmed.
In practice, it is usually very difficult to provide edge trimming cutters and to discharge the trimmed edges efficiently using a single roller system, due to a shortage of space. Only the edges of the finished profile can be trimmed, that is to say, the edges of the tread strip which is extruded from the plurality of different rubber mixtures.
Such edge trimmings cannot be re-utilised because they comprise mixtures of rubber having substantially different properties. Thus, for example, the rubber used for giving good wear resistance to the tread strip has a very high carbon content while the rubber used for the inner ply of the tread strip is very soft.
In consequence, several tons of scrap edge strips are produced every day. Such scrap cannot be re-used or, at best, can only be used for the manufacture of very inexpensive, low quality rubber articles.
The edges of the rubber mixtures, which overlap one another in the centre of the tyre profile, cannot be trimmed because it is only possible to trim the edges of the finished tread strip. In consequence, considerable inaccuracies have to be tolerated in respect of the finished tire profiles produced on such a single roller head system. This, inevitably, causes weak spots to be produced in the finished automotive vehicle tire. It is also not possible to trim the edges of the individual tread strip components when such a head is employed.